Different modes of loss of photoreversibility of ultraviolet light-induced true and suppressor mutations to tryptophan independence in an auxotrophic strain of Escherichia coli

Abstract

The loss of photoreversibility of UV-induced mutations to tryptophan independence in a tryptophan-requiring strain of E. coli (left after “ mutation frequency decline”, produced by incubation in the absence of the amino acid, has eliminated most suppressor mutations) occurs during the first 20 min of postirradiation incubation. It is amino acid independent but is blocked by dinitrophenol. It is independent of DNA replication since it occurs in the presence of nalidixic acid, an inhibitor of DNA replication. Loss of photoreversibility of the suppressor mutations (subject to mutation frequency decline) occurs in correlation with the initial doubling of DNA in the culture and may be prevented by nalidixic acid, suggesting that DNA replication is required for such loss. The initial postirradiation replication of DNA is significant to the mutation process since expression of both sorts of mutation (measured on tryptophan-free minimal agar medium) does not occur unless DNA synthesis is allowed before plating and is prevented by nalidixic acid. The data are consistent with the hypothesis that dimer excision accounts for loss of photoreversibility of stable reversions to tryptophan independence but that DNA replication is necessary to this process for suppressor mutations of this requirement subject to mutation frequency decline.